In multiphoton induced curing processes, which are described in, for example, U.S. Pat. No. 6,855,478, incorporated herein by reference, a layer including a multiphoton curable photoreactive composition is applied on a substrate such as, for example, a silicon wafer, and cured using a focused light source such as a laser beam. The multiphoton curable photoreactive composition in the applied layer includes at least one reactive species that is capable of undergoing an acid or radical initiated chemical reaction, as well as a multiphoton initiator system. Imagewise exposure of the layer with light of an appropriate wavelength and sufficient intensity causes two-photon absorption in the multiphoton initiator system, which induces in the reactive species an acid or radical initiated chemical reaction in a region of the layer that is exposed to the light. This chemical reaction causes crosslinking, polymerization or a change in solubility characteristics in the exposed region, referred to herein as curing, to form a cured object. Following the curing step, the layer may optionally be developed by removing a non-cured portion of the layer to obtain the cured object, or by removing the cured object itself from the layer.
The layer including the multiphoton curable photoreactive composition typically has a relatively uniform thickness of about 10-500 μm, and curing can occur at any location within this layer to form a cured object. However, to make sure the cured object is attached to the substrate, the curing process should be started at an interface between the curable composition and the substrate. The accuracy with which this interface should be located varies widely depending on the particular cured structure that is to be made, but typically the interface should be located within a range of about 100 nm to 1 μm.
Conventional multiphoton curing processes have utilized a surface mapping technique in which the entire substrate surface is mapped in advance of the curing step to locate the interface between the substrate and the layer including the multiphoton curable photoreactive composition. In the alternative, a second light beam, different from the light beam used to cure or initiate cure in the layer including the multiphoton curable photoreactive composition, has been used to track the changes in the substrate surface and locate this interface. However, these techniques do not take into account environmental changes and process variations that occur between the time the surface measurement is made and the initiation of the curing process, or while the curing process is underway. As a result, the interface location information these processes provide is not sufficiently accurate, particularly in a process used to make large cured objects with a size of greater than about 1 cm2.